Spring is here, and you probably want to get outside. But an unseen danger could be lurking in the green grass or your hiking trail.

While it sounds like the opening of a horror movie, it's all too real. The deer ticks that carry Lyme disease come out in the spring, and they're hungry and looking for a blood meal.

Ever since it was discovered in Connecticut, Lyme has been spreading across the country. It’s now the number one insect-transmitted disease in North America.

While many still believe Lyme disease is confined to the Northeast, Lyme-carrying ticks have invaded the Midwest, California, and Canada, and have become a serious public health threat.

Stanford researchers were surprised to discover ticks infected with Lyme disease in just about every park studied in the San Francisco Bay Area. Earlier studies by the University of California also found squirrels in California to be infected by the Lyme bacterium.

Deer ticks that carry Lyme disease are also increasing in Illinois and Indiana, after having been found in Wisconsin and Minnesota. While these ticks thrive in forested areas, living on deer and mice, they're also spotted on voles in the prairie, a sign that they're moving into new habitats and expanding their range.

Now Lyme-carrying deer ticks are not only showing up in unlikely places, but they're also carried by a surprising list of animals. Deer ticks hitch a ride on deer, white-footed mice, chipmunks, shrews, prairie dogs, squirrels, raccoons, opossums, skunks and even birds.

Early symptoms of Lyme include fever, headache, fatigue and sometimes a telltale rash that looks like a bull's-eye centered on the tick bite. Chronic Lyme can lead to arthritis, joint pain, immune deficiencies, a persistent cognitive issues and other neurological problems.

Lyme can be difficult to diagnose. In the U.S. alone, the CDC revised in 2013 the estimated number of Americans diagnosed with Lyme disease every year to be around 300,000, roughly 10 times higher than the previously reported.